The present invention relates to a power supply circuit using a battery as a power supply and raising the voltage of this battery, and more particularly relates to a power supply circuit for a wireless transmitter which keeps supplying power to a load for a prescribed period of time after a switching off operation of the power supply circuit.
In recent years, the progress towards compact, portable electronic equipment has been accompanied by the widespread need for power supplies to be comprised of a single battery. However, in spite of developments which have been enabling electronic components or IC's etc. to be operated with a voltage of a single battery, few electronic equipments are capable of controlling all of their operations with a low voltage of, for example, 1.5 volts. Consequently, power supplying in most of such equipments has been carried out by boosting this low voltage up to the high-voltage required for their circuits.
Further, electronic equipment also exists where, when the power supply is turned off after use, it is necessary for the circuits to be made to operate for a prescribed time so that all circuit operations can be completed. For example, in the case of a wireless transmitter, when use is over, a signal indicating that "the transmission side power supply is OFF" is sent from the receiving side when the power supply is turned OFF, the receiver audio mute circuit is made to operate and the occurrence of unnecessary noise is prevented, i.e. after the power supply is OFF, a voltage is maintained for a prescribed period of time and the transmitter is made to operate, and demands for power supplies having a function where the voltage is maintained for a prescribed period of time after the power supply is OFF are therefore increasing.
Next, the aforementioned related power supply circuit example is described with reference to FIG. 1 and FIG. 2.
First, in a first related example shown in FIG. 1, a transistor Q11 is provided across a battery 4 and a DC-DC converter 13, with the DC-DC converter 13 being controlled by the transistor Q11.
When the power supply is on so that a power supply switch S11 is turned ON, a capacitor C11 starts to charge, with a charge voltage VC11 finally reaching a battery voltage EQ. Further, a base voltage VB of a transistor Q12 becomes the voltage VC11 divided by a resistor R13 and a resistor R14 (the internal resistance of the battery is considered to be 0 for simplicity) in a manner such that the voltage VB becomes: EQU VB=VC11.multidot.R14/(R13+R14) (1).
Here, the transistor Q12 becomes ON when this base voltage VE exceeds a voltage VBE across the base and emitter of the transistor Q12, current is drawn out from the transistor Q11, transistor Q11 is made to go ON, and a voltage is supplied to the DC-DC converter 13. This supplied voltage is then converted to a prescribed voltage and supplied to the load circuit.
Next, when the power supply switch S11 is put OFF in order to break the power supply, the discharge of the load stored at the capacitor C11 starts via resistors R13 and R14. At this time, the discharge voltage VC11 becomes: EQU VC11=E0exp-1/C11(R13+R14)!t (2),
and the base voltage of the transistor Q12 becomes the voltage VC11 divided by resistor R13 and resistor R14 so as to become: EQU VB=E0exp-1/C11(R13+R14)!t.times.R14/(R13+R14) (3).
The transistor Q12 then goes off when this base voltage VB becomes lower than the voltage VBE across the base and emitter of transistor Q12 after a time t1. The transistor Q11 therefore also becomes OFF and the operation of the DC-DC converter 13 is halted.
Namely, at the power supply circuit of the aforementioned configuration, after the power supply switch S11 goes OFF, the operation of the DC-DC converter 13 continues until the passage of time t1, and the supplying of power to the load circuit is possible.
Here, it is necessary for a base current of Ib to flow in order to make the transistor Q11 functioning as an electronic switch go ON. However, it is necessary for the relationship: EQU Ib.times.hfe&gt;Ic (4)
hfe: current amplification factor to be fulfilled in order for a sufficient current Ic to flow under stable conditions.
When the DC-DC converter 13 is activated, a rush current several times larger than under normal conditions flows and a base current Ib therefore has to be set to maintain this activation. Further, because hfe fluctuates a great deal, this margin has to be maintained and a base current Ib having a margin with respect to the temperature characteristics has to be made to flow. This effectively usually means that a null current flows and power supply efficiency is lowered.
Next, in a second related example shown in FIG. 2, the transistor Q11 of the first related example is replaced with a Field Effect Transistor (hereinafter referred to simple as "FET") Q21, with other aspects of the configuration and operation then being the same as the first related example. An FET is a voltage-controlled element and control current can therefore be reduced when compared with transistors. The null current is therefore lowered and the efficiency of the power supply can therefore be improved. There is, however, a problem whereby efficiency deteriorates if the ON resistance of the FET is made large so that the input voltage of the DC-DC converter falls. It is therefore difficult to drive current general-purpose FETs using a voltage from a single battery because the VGS ON voltage of current general-purpose FETs is high.
In the aforementioned related example, a transistor Q11 or an FET Q21 for switching use is added to a primary side input path but resulting voltage drops cannot be prevented and power supply efficiency deteriorates, particularly when the primary power supply voltage is low.
A configuration where a battery is directly connected to the primary side input can also be considered. In this case, efficient utilization of the input power is possible but there is the fear of current leaking to the secondary circuitry. Namely, when rectification is carried out by a simple diode, a current path can still be made from the battery 4 to the secondary circuitry via the diode as a result of the voltage of the battery 4 even when the power supply switch 11 is OFF and the operation of the DC-DC converter 13 is halted. When the power supply switch S11 is OFF over long periods of time the battery 4 is consumed quickly.
It is therefore the object of the present invention to provide a power supply circuit capable of efficiently raising the efficiency of a low voltage of a battery etc. used as a power supply in electronic equipment for use with load circuitry within the electronic equipment operating at a higher voltage rather than the low voltage and capable of supplying a voltage to the load circuitry for a prescribed period of time after the power supply has been switched off.